The present invention relates to protective glass-ceramic coatings and to refractory structural articles provided with the protective coatings. More specifically, the invention pertains to glass-ceramic coatings useful for the protection of oxidizable refractory substrate materials such as refractory intermetallic aluminides, titanium alloys, carbon-carbon composites, and iron, cobalt or nickel-based superalloys. The coatings provide a chemical barrier which is effective to protect the substrate from oxidation or other physical or chemical deterioration at high temperatures in hostile environments.
There is a continuing need for materials offering good strength and toughness as well as excellent resistance to chemical attack at very high use temperatures. In the aerospace industry, for example, refractory materials such as carbon-carbon composites, superalloys, and intermetallic titanium aluminide compounds are being increasingly used for airframe, engine and other components for supersonic and advanced ultrasonic aircraft such as the national aerospace plane (NASP).
Aluminide intermetallic compounds, which are candidates for large structural components of the space plane, offer a unique combination of low density, high strength, and sustained resistance to temperatures up to 1000.degree. C. However, each of the two aluminide candidate materials presently being evaluated, i.e., titanium aluminide in the gamma form (TiAl) and in the super .alpha.-2 form (Ti.sub.3 Al), are susceptible to oxidation attack as well as to hydrogen embrittlement at elevated temperatures.
Carbon is one of the most refractory elements known, with a melting point of over 4000.degree. C., and new composite materials composed of carbon reinforced with carbon fibers offer the highest strength/weight ratios of any known materials. Unfortunately, the reactivity of carbon with oxygen to form CO.sub.2 at temperatures below about 600.degree. C. severely limits the usefulness of carbon-based materials in oxidizing environments. The promise of a carbon-carbon composite that could be protected from oxidation to make full use of its high strength/weight ratio and melting point has attracted great interest in the aerospace community.
Protective coatings of silicon carbide, applied by various techniques such as direct solid-state reaction, sputtering, evaporation, or the like, are known to provide limited protection to carbon composite materials at temperatures in the 1000.degree. C. range, but cracking and oxidation remain persistent problems. Boron oxide, alone or in combination with other oxides, has been incorporated in the these substrates and/or the coatings to improve crack resistance, but the resulting materials characteristically melt at low temperatures and lose their effectiveness at temperatures much above 1000.degree. C.
Also benefitting from protective coatings are metallic components composed of the so-called superalloys. These alloys are typically iron-, nickel- or cobalt-based alloys comprising substantial proportions of chromium, iron, cobalt or nickel as alloying constituents. Although more oxidation-resistant than carbon composite materials, these alloys are also subject to high temperature oxidation, and therefore have also been protectively coated for increased high-temperature protection.
A conventional method for protecting refractory materials such as described from oxidation at elevated temperatures is to apply a continuous monolithic glass coating thereto. Glass can completely encapsulate and isolate a protected substrate from the surrounding atmosphere; however, glass layers are subject to erosion or displacement by viscous flow at high temperatures.
The high temperature viscosity of glass coatings may be increased by mixing crystalline materials with the glass frits before application of the coating. However, these glass-crystalline mixtures sinter non-uniformly, and the crystal size, homogeneity, and flow of the coatings are thus very difficult to control.
Polycrystalline ceramic coatings have been proposed as a way to protect superalloy materials from oxidative deterioration. Thus U.S. Pat. Nos. 4,485,151 and 4,535,033 (Stecura) describe the application of insulating layers of stabilized ZrO.sub.2 to such materials via a plasma-spraying technique.
Ceramic plasma spraying procedures can involve several steps that are tedious and difficult to control in commercial production. Further, thermal gradients tend to develop during plasma-spraying which introduce defects in the finished coating, and the finished coatings tend to be porous. This permits access of gases, in particular O.sub.2, H.sub.2, SO.sub.2, and water vapor, all of which can contribute to coating failure.
Glass-ceramic materials are of course well known and a wide variety of glass-ceramic compositions for various uses, including coatings, has been developed. U.S. Pat. No. 3,397,076 (Little et al.), for example, describes fused crystallizable ground and cover coats for high temperature alloys in which the major elements are cobalt, nickel, chromium, iron or mixtures. The ground coat is lithium-free and contains 35-65% SiO.sub.2 and 12-45% BaO. Examples also contain substantial amounts of R.sub.2 O, B.sub.2 O.sub.3 and/or TiO.sub.2.
U.S. Pat. No. 3,467,534 (MacDowell) discloses glass-ceramic articles consisting essentially of 20-70% BaO and 30-80% SiO.sub.2 and having a barium silicate principal crystal phase. A preferred example is described as considered for coating metals. U.S. Pat. No. 4,861,734 (MacDowell) discloses alkaline earth aluminoborate glass-ceramics, produced through a process of sintering finely-divided borate glasses of appropriate composition, which exhibit relatively high levels of crystallinity and dielectric properties rendering them suitable for applications such as integrated circuit packaging.
Notwithstanding the fact that both glass-ceramic and glass coating technologies are highly developed, there remains a need for new protective coating formulations which could protect refractory carbonaceous, metallic, and intermetallic surfaces from oxidation or other deterioration at high temperatures. It is accordingly a principal object of the present invention to provide protective coatings of improved integrity and refractoriness for use in the protection of such substrates.
Another object is to provide such coatings which are both more effective than previously known coatings and more convenient to apply.
A further object is to provide protective coatings which are non-porous, continuous and free from defects such as pinholes and cracks, and thus a barrier to the diffusion of oxygen as well as hydrogen and other corrosive gases.
A still further object is to provide a barrier coating that adheres tightly to various refractory substrates and resists spalling during thermal cycling.
Another object is to provide an oxygen barrier coating material that exhibits the excellent flow characteristics of a glass coating as it is fired in one temperature range, and becomes resistant to flow (due to crystallization) as it is heated in a higher temperature range.
A further object is to provide a protectively coated article comprising a substrate portion composed of a carbon composite, intermetallic or metal alloy and a protective glass-ceramic coating offering improved resistance to mechanical abrasion and chemical corrosion, adherence, refractoriness, integrity and/or permeation resistance than prior art coatings.